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41.
Seismic observations in the DPRI 1800 m borehole drilled into the Nojima Fault zone, south-west Japan 总被引:2,自引:0,他引:2
Abstract Seismometers were installed at three depths in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m borehole drilled into the Nojima Fault zone, southwest Japan. The waveforms recorded by these seismometers are rather simple compared with those recorded at the DPRI 800 m borehole or on the ground surface. These data should be well suited for detecting fault zone-trapped waves and estimating the fault zone structure and its temporal variation related to the healing process of the ruptured fault. Typical waveforms trapped in the fault zone were observed by a surface seismographic array across the Nojima Fault just after the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). Among the wave data recorded in the DPRI 1800 m borehole, however, clear evidences of fault zone-trapped waves have not yet been found, and further studies are continuing. The present study outlines the observation system in the DPRI 1800 m borehole, which will make it easier to access and analyze the borehole data. 相似文献
42.
Partition of Fe2+ and Mg between coexisting (Mg, Fe)2SiO4 spinel and (Mg, Fe)SiO3 pyroxene was investigated at pressures 80 and 90 kbar and at temperatures 840 and 1050° C, using tetrahedral-anvil type of high pressure apparatus. Olivine-spinel solid solution equilibria in the system Mg2SiO4-Fe2SiO4 were discussed in the light of the partition reaction. Partition of Fe2+ and Mg in both olivine-spinel and pyroxene-spinel systems can not be regarded as that between ideal solid solutions. By applying the simple solution model for the partition of Fe2+ and Mg, sign of the heat of mixing was estimated to be positive for all olivine, spinel and pyroxene. Relative concentration of Fe2+ in spinel in the pyroxene-spinel system is likely to cause some change in the chemical composition of modified spinel () or spinel () in the transition zone of the mantle. A considerable change is also expected in the transition pressure of to ( + ) and ( + ) to .Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrology. 相似文献
43.
A. Schubnel O. Nishizawa K. Masuda X. J. Lei Z. Xue Y. Guégen 《Pure and Applied Geophysics》2003,160(5-6):869-887
— A set of experiments on four samples of Oshima Granite at 15, 40 and 60 MPa confining pressure have been performed in order to investigate the damage behavior of granite submitted to deviatoric stress. In addition an experiment on one sample of Toki Granite at 40 MPa confining pressure was performed, in order to compare and elucidate the structural effects. Using acoustic emission data, strain measurements and elastic wave velocities allow to define consistently a damage domain in the stress space. In this domain, microcracking develops. The microcracking process is, in a first stage, homogeneous and, close to failure, localized. Elastic wave velocities decrease in the damage domain and elastic anisotropy develops. Using Kachanov's model (1993), elastic wave velocities have been inverted to derive the full second-order crack density tensor and characterize the fluid saturation state from the fourth-order crack density tensor. Crack density is strongly anisotropic and the total crack density close to failure slightly above one. The results indicate that the rock is saturated in agreement with the experimental conditions. The model is thus shown to be very appropriate to infer from elastic wave velocities a complete quantitative characterization of the damaged rock. 相似文献
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Morphology and tectonics of the Yap Trench 总被引:5,自引:0,他引:5
Fujiwara Toshiya Tamura Chiori Nishizawa Azusa Fujioka Kantaro Kobayashi Kazuo Iwabuchi Yo 《Marine Geophysical Researches》2000,21(1-2):69-86
We conducted swath bathymetry and gravity surveys the whole-length of the Yap Trench, lying on the southeastern boundary of the Philippine Sea Plate. These surveys provided a detailed morphology and substantial insight into the tectonics of this area subsequent the Caroline Ridge colliding with this trench. Horst and graben structures and other indications of normal faulting were observed in the sea-ward trench seafloor, suggesting bending of the subducting oceanic plate. Major two slope breaks were commonly observed in the arc-ward trench slope. The origin of these slope breaks is thought to be thrust faults and lithological boundaries. No flat lying layered sediments were found in the trench axis. These morphological characteristics suggest that the trench is tectonically active and that subduction is presently occurring. Negative peaks of Bouguer anomalies were observed over the arc-ward trench slope. This indicates that the crust is thickest beneath the arc-ward trench slope because the crustal layers on the convergent two plates overlap. Bouguer gravity anomalies over the northern portion of the Yap Arc are positive. These gravity signals show that the Yap Arc is uplifted by dynamic force, even though dense crustal layers underlie the arc. This overlying high density arc possibly forces the trench to have great water depths of nearly 9000 m. We propose a tectonic evolution of the trench. Subduction along the Yap Trench has continued with very slow rates of convergence, although the cessation of volcanism at the Yap Arc was contemporaneous with collision of the Caroline Ridge. The Yap Trench migrated westward with respect to the Philippine Sea Plate after collision, then consumption of the volcanic arc crust occurred, caused by tectonic erosion, and the distance between the arc and the trench consequently narrowed. Lower crustal sections of the Philippine Sea Plate were exposed on the arc-ward trench slope by overthrusting. Intense shearing caused deformation of the accumulated rocks, resulting in their metamorphism in the Yap Arc. 相似文献
47.
Michihiko Nakamura Keiichi Otaki Shingo Takeuchi 《Journal of Volcanology and Geothermal Research》2008
The relationship between permeability and vesicularity in volcanic rocks has been used to infer the degassing behavior of hydrous magma. Recent data on natural samples from various eruptions show a wide variation, fitting a power–law relationship of the percolation models with low (< 30%) critical vesicularity (ФC). In this study, we present data on permeability and pore-connectivity of juvenile rhyolitic pumice clasts in a pyroclastic flow around Onikobe volcano, NE Japan, and investigate their relationship with vesicularity developed in a single eruption event. The permeability of the pumices having a relatively low abundance of microlites and microphenocrysts shows a trend increasing by 4 orders of magnitude (from 10− 13.8 to 10− 10.1 m2) in a high and narrow vesicularity range (from 72 to 80%). This trend intersects at a high angle with the fit to the permeability–vesicularity data in the previous studies that has a low ФC, and is located on the extension of the trend for the products of isotropic decompression experiments. The two-dimensional (2D) connectivities of pores for the pumices were also measured from thin sections. From the point of view of percolation theory, connectivity provides information about the probability of percolation. They showed a steep increase from ca. 0 to 0.7 in an almost similar vesicularity range, as compared to their permeabilities. We attribute the increase in 2D connectivity to the increasing amount of ruptured bubble walls, which might have provided less-tortuous paths through larger apertures for gas flow. This, in turn, would cause an effective increase in the permeability. Aggregates of bubble-wall-shaped glass shards were found in the pumices, and their amount and degree of welding are higher in the pumices that have a higher abundance of microlites and microphenocrysts. These pumices have relatively high permeability and 2D connectivity at low vesicularity, which is accounted for by the existence of large irregularly shaped pores. These textural characteristics suggest that a series of partial fragmentation processes, including local rupturing of bubble walls and subsequent foam-collapse with permeable gas flow, might have occurred before the ultimate bulk fragmentation, thus resulting in the increase in permeability. We suggest that the 2D connectivity of pores is a useful parameter to quantify the degree of fragmentation of bubble walls and has the potential for use to assess their permeability. 相似文献
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A Middle Pleistocene widespread tephra, defined here as Hegawa-Kasamori 5 tephra (Hgw-Ks5), has been newly recognized over a broad area of Japan. Large-scale pyroclastic flow deposits associated with co-ignimbrite ash fall deposits (CAFDs) of Hgw-Ks5 have been identified in the proximal southern Kyushu area, south-west Japan. Hgw-Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north-west area of the Aira Caldera, the tephra is sparsely distributed in the form of non-welded ignimbrites, and is exposed stratigraphically above the well-known Kobayashi-Kasamori tephra. Hgw-Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw-Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw-Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km3. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw-Ks5 eruption. 相似文献
50.
Evaluation of slope stability by finite element method using observed displacement of landslide 总被引:1,自引:0,他引:1
To monitor land deformation in detail, we ran a large-scale field test in which an artificial landslide was induced by the application of a load to a natural slope. The measured landslide displacement was reproduced numerically through the use of finite element model analysis with a two-dimensional elasto-viscoplastic model. The analysis suggested that the strength of the sliding surface decreased as the landslide mass moved. We propose a simple method for estimating safety factors. The method involves back-calculation of shear strength parameters through reproduction of observed landslide displacements and calculating the ratio of driving force to resisting force acting on the sliding surface as modeled by joint elements. This ratio, the “stability index”, shows the same trend as safety factors calculated by a two-dimensional limit equilibrium method and a shear strength reduction method that use back-calculated shear strength parameters estimated from the limit equilibrium state. The results indicate that the stability index may be applicable to the assessment of slope stability. 相似文献